In-situ leaching of mineral values from an ore body has been used for many years in the mining industry, particularly in the production of uranium. Generally, a leaching solution or lixiviant is pumped under pressure into the ore body through one or more injection wells. The lixiviant percolates and migrates through the ore body and solubilizes the desired mineral values. The various chemical processes used for this purpose are well described in the literature. The pregnant lixiviant is removed from the ore body through one or more production wells for subsequent processing to extract the solubilized minerals.
One common problem with in-situ leaching has been confinement of the lixiviant within the desired portion of the ore body. Although the pressure differential between the injection and production wells tends to cause the lixiviant to migrate through the ore body toward the production wells, some of the lixiviant will migrate beyond the remaining portions of the ore body and into surrounding formations. This loss of lixiviant is not only an economic loss to the mine operator, but also may result in ground water contamination.
In response to this problem several methods have been developed in the past to produce an impermeable barrier to confine the lixiviant, as shown in the following prior art references:
______________________________________ Inventor U.S. Pat. No. Issue Title ______________________________________ Lyons 4,311,340 1/19/82 "Uranium Leaching Process and Insitu" Fehlner 3,819,231 6/25/70 "Electrochemical Method of Mining" Zakiewicz 4,289,354 9/15/81 "Borehole Mining of Solid Mineral Resources" ______________________________________
The Lyons patent most clearly demonstrates the concept of completely encapsulating the ore body. Lyons also teaches use of vertical boundary wells (FIGS. 1-4) to form a vertical curtain of impermeable material around the ore body, as is also shown by Felner. Lyons also teaches that hydrofracturing of these boreholes may be employed to create cracks and passageways in the strata surrounding the boreholes to facilitate greater penetration of the grout or other impermeable materials (columns 7-8). Finally, Lyons discloses that organic polymers and epoxy resins, as well as a wide variety of other materials can be used to create this impermeable barrier.
The primary limitation of Lyons is the manner in which the horizontal barriers are formed above and below the ore body. Lyons relies on slanted boreholes formed by directional drilling for this purpose, as shown in FIGS. 5-11. While this technique may be effective for a relatively small ore body, it quickly becomes impractical when dealing with a large ore body, particularly one having a large horizontal cross-section. In such cases, a radial arrangement of slanted boreholes does not result in a uniform degree of encapsulation of the ore body due to radial diversion of the boreholes. Directional drilling also entails additional costs. Finally, the method disclosed by Lyons is best suited for situations where the top and bottom surfaces of the ore body are regular in contour. In contrast, the present invention eliminates these disadvantages by forming the horizontal barriers as part of the process of completing the injection and production wells.